I'm so glad your mastitis has cleared up 
Was the midwife an infant feeding specialist? Did she assess your DD's tongue function by putting a finger in her mouth and observing whether she could move her tongue properly?
"Assessment for tongue-tie requires training and skill and involves placing a finger in the baby's mouth. It cannot be done by just taking a look. Assessment is usually carried out with the baby on the assessor's lap or a flat surface such as a table or couch. It involves observing how the baby uses their tongue.
Professionals assessing babies for tongue-ties should assess elevation, lateralisation and extension. Elevation can most easily be assessed when baby cries. With the mouth wide open, the tongue tip should lift up to at least the mid mouth. In tongue-tied babies the tongue often stays quite flat in the floor of the mouth or the edges curl up to form a bowl shape or 'v' shape. Babies should be able to poke their tongue tip out well over the bottom lip when the bottom lip is stimulated. When the assessor runs their finger along the top ridge of the bottom gum the tip of the baby's tongue should follow the finger so the tongue sweeps side to side (lateralisation).
Some assessors perform a suck assessment by placing their finger in the baby's mouth (pad side up, nail side down) and feeling how the baby is cupping and using their tongue. Assessors sweep their finger under the baby's tongue so they can feel the extent of the tongue tie and the tongue will also be lifted to visualise the frenulum. The appearance of the frenulum is also documented including the shape of the tongue tip, where it attaches to the floor of the mouth and the underside of the tongue and how long and stretchy it is."
(Copied and pasted from the page I linked to in my earlier post.)
If she did all of the above and said no TT, fair enough. But if she didn't assess tongue function I would suggest getting a second opinion.
My DS was gaining weight despite the TT, he was just feeding pretty constantly in order to do so, as he wasn't very efficient.
Oh and I have heard good things about cranial osteopathy so that could be worth a try. We took DS to an osteopath, although it was the TT division that really made a difference in our case, I guess it depends what the issue is.
Were there any complications at the birth?